Microsoft Office Outlook 2003 can support much larger Personal Folders (.pst) files than earlier versions--up to 33TB. Given the possibility that archive or export .pst files might grow large enough to crowd out other data on users' hard disks, you might want to look again at .pst file usage in your organization. Outlook 2003 lets you use Group Policy to control the size of both the old and new .pst files. Even if you aren't planning to move to Outlook 2003 any time soon, knowing how Outlook uses .pst files and which uses you can disable in earlier versions can come in handy.

Outlook uses .pst files in more ways than you might realize. The most familiar use is a standalone user using a .pst file instead of an Exchange mailbox as the default information store. Users typically can create additional .pst files by turning on Outlook's AutoArchive feature, which moves older items into a .pst file on a regular schedule, or by using the File, Import and Export command to export data to a .pst file. Outlook also automatically creates a .pst file if the user adds an IMAP4 or Hotmail account to his or her email profile; Outlook uses this .pst file to store a local cache of the IMAP or Hotmail account's messages. Finally, Outlook 2003 creates a new .pst file if the user views a Windows SharePoint Services (WSS) events or contacts list and clicks the "Link to Outlook" link; this .pst file contains the local copy of the linked SharePoint lists.

To provide some control over .pst file use, Outlook 98 and later support a DisablePST registry value. This DWORD value is under the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\11.0\Outlook registry subkey in Outlook 2003; change "11.0" to "10.0" for Outlook 2002, "9.0" for Outlook 2000, or "8.0" for Outlook 98. The DisablePST value isn't present by default, so you need to add it if you want to disable .pst file use. Allowable values are 1 (disabled) and 0 (enabled, the default).

DisablePST's initial use in early builds of Outlook 2002, Outlook 2000, and Outlook 98 was to prevent users from archiving and exporting to or importing from a .pst file. Setting DisablePST to 1 in those versions turns off those features and disables the creation of new .pst files from the File, New menu. User accounts that you configure in Corporate/Workgroup mode, however, can still add a new or existing .pst file to their email profile (through the File, Data File Management dialog box in Outlook 2002 or the Tools, Services dialog box in Outlook 2000 or Outlook 98), so some ad hoc .pst file use is still possible in these versions.

Beginning with Outlook 2002 (with Office XP Service Pack 2--SP2--applied) and continuing with Outlook 2003, DisablePST blocks all user-controlled methods of creating a new .pst file. Setting DisablePST to 1 prevents all new .pst files except those that Outlook itself creates for use with IMAP or Hotmail accounts or WSS lists.

Even with the archive and export features disabled, however, users can open an existing .pst file by using the File, Open, Outlook Data File command, and users can copy or move items into that file. If you want to prevent users from opening a .pst file, you can use Group Policy to disable the Outlook Data File command. You'll need to know the command's ID, which is 5576, and be familiar with Group Policy operations. (For Group Policy resources, search the Windows & .NET Magazine Web site at http://www.winnetmag.com .)

When you deploy Outlook 2003, you can use the Custom Installation Wizard or Custom Maintenance Wizard from the Office Resource Kit (ORK) to determine what kind of .pst files users can create. On the wizards' Change Office User Settings page, look for the PST Settings options under Microsoft Office Outlook 2003/Miscellaneous. You can set values for "Default location for .pst files" and "Preferred PST Mode (Unicode/ANSI)." A Unicode .pst file is the new type supported in Outlook 2003; ANSI refers to the older type that's limited to 2GB. Setting the Preferred PST Mode adds a string registry value named NewPSTFormat to the HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook subkey. NewPSTFormat can have the following values:

Prefer Unicode PST: 0 (default)
Prefer ANSI PST: 1
Enforce Unicode PST: 2
Enforce ANSI PST: 3

By using a value of 2 or 3, you can restrict all new .pst files to either the old format or the new format.

The corresponding set of policies listed in Group Policy Editor (GPE) under User Configuration\Administrative Templates\Microsoft Office Outlook 2003\Miscellaneous\PST Settings goes one step further than the ORK tools and provides a way to limit the growth of both old and new .pst files without totally restricting their use. A bit inconsistently, the policies use the term "Large PST" for the Unicode format and "Legacy PST" for the older ANSI format, but for each .pst format, you can set an absolute maximum size and a separate "Size to disable adding new content." The default maximum size for a Unicode .pst file is about 20GB.

With this array of policies and registry settings, Outlook--particularly Outlook 2003--lets you control end users' access to .pst files, set .pst file size limits, control the type of .pst files Outlook 2003 users can create, prohibit the use of new .pst files, and even suppress the menu command for opening an existing .pst file.